Add dihedrals

[AntonOresten]

Based on #8 (i don't really know how pull requests, forks, and remotes work)

Changes to #8:

• Access the batched_mul function through NNlib instead of Flux to avoid unnecessary indirect dependencies
• Rename backbone/distance.jl to backbone/bonds.jl
• Use backbone_bond_vectors function in backbone/bonds.jl
• Add tests for idealized bond length functions (tests still missing for some functions)
• Rename global bond_lengths and bond_angles variables to MEAN_BOND_LENGTH and MEAN_BOND_ANGLE turn into consts.
• Remove module Dihedrals with hopes of making the functions work well with the Backbone type
• Turn bond_lengths and bond_angles into keyword arguments
• Rename get_triangle to law_of_cosines
• Add ::Backbone method to get_dihedrals function

[codecov]

Codecov Report

Attention: 53 lines in your changes are missing coverage. Please review.

Comparison is base (23e783f) 100.00% compared to head (47edee6) 84.31%.

Files	Patch %	Lines
src/backbone/dihedrals.jl	59.54%	53 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              main       #9       +/-   ##
============================================
- Coverage   100.00%   84.31%   -15.69%     
============================================
  Files           10       11        +1     
  Lines          197      338      +141     
============================================
+ Hits           197      285       +88     
- Misses           0       53       +53     

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